Make VFAT filesystem from Go

write_vfat.go

package main

import (
	"encoding/binary"
	"fmt"
	"io"
	"os"
)

const bytesPerSector = 512

func main() {
	err := do()
	if err != nil {
		fmt.Println(err)
	}
}

func do() error {
	f, err := os.Create("made_vfat.img")
	if err != nil {
		return err
	}
	defer f.Close()

	//// BIOS Parameter Block

	// Jump to boot code
	f.Write([]byte{0xeb, 0x58, 0x90, 'M', 'S', 'W', 'I', 'N', '4', '.', '1'})

	// Basic parameter block
	write(f, uint16(bytesPerSector))
	write(f, uint8(1)) // 1 sector per cluster
	write(f, uint16(2)) // 2 reserved sectors
	write(f, uint8(1)) // 1 FAT
	write(f, uint16(0)) // 0 directory entries
	write(f, uint16(4)) // 4 sectors in this volume
	write(f, uint8(0xf8)) // This is a "fixed disk", rather than a floppy disk
	write(f, uint16(0)) // Not used on FAT32 (Sectors per FAT, on FAT16)
	write(f, uint16(1)) // 1 sector per track (dummy value)
	write(f, uint16(64)) // 64 heads (dummy value)
	write(f, uint32(0)) // 0 sectors before partition, since we have no partitions
	write(f, uint32(0)) // "Large sector count" not used; used small count above

	//// Extended Boot Record

	// Extended BIOS Parameter Block
	write(f, uint32(1)) // FAT is 1 sector long
	write(f, uint16(0)) // No flags
	write(f, uint16(0)) // FAT version number
	write(f, uint32(2)) // Root directory is in cluster 2
	write(f, uint16(1)) // FSInfo structure is in sector 1
	write(f, uint16(0)) // Backup boot sector is the real boot sector (no backup)
	f.Write([]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0})
	write(f, uint8(0x80)) // This is a hard disk
	write(f, uint8(0)) // No flags
	write(f, uint8(0x29)) // Signature
	write(f, uint32(0)) // Serial number
	f.Write([]byte{'H', 'e', 'l', 'l', 'o', ' ', ' ', ' ', ' ', ' ', ' '})
	f.Write([]byte{'F', 'A', 'T', '3', '2', ' ', ' ', ' '})

	// Skip over the "boot code"
	seek(f, 0, bytesPerSector - 2)

	// Bootable partition signature
	// (We're not actually bootable so maybe we should't set this?)
	write(f, uint16(0xAA55))

	//// FS Information Sector
	seek(f, 1, 0)
	f.Write([]byte{0x52, 0x52, 0x61, 0x41}) // Signature
	seek(f, 1, 0x1e4)
	f.Write([]byte{0x72, 0x72, 0x41, 0x61}) // Signature
	write(f, uint32(0xffffffff)) // Unknown number of free data clusters
	write(f, uint32(0xffffffff)) // Unknown most recently allocated cluster
	seek(f, 1, 0x1fc)
	f.Write([]byte{0x00, 0x00, 0x55, 0xaa}) // Signature

	//// File Allocation Table
	seek(f, 2, 0)
	f.Write([]byte{0xf8, 0xff, 0xff, 0x0f}) // Signature
	f.Write([]byte{0xff, 0xff, 0xff, 0x0f}) // End of chain marker
	f.Write([]byte{0xff, 0xff, 0xff, 0x0f}) // Root directory ends here
	f.Write([]byte{0xff, 0xff, 0xff, 0x0f}) // HELLO.TXT ends here

	//// Root Directory Table
	filename := []byte("HELLO   TXT")
	seek(f, 3, 0)
	// Long filename entry
	write(f, uint8(0x41))
	f.Write([]byte{'H', 0, 'e', 0, 'l', 0, 'l', 0, 'o', 0})
	write(f, uint8(0xf)) // "Long filename" attribute
	write(f, uint8(0x0)) // Reserved
	checksum := uint8(0)
	for _, b := range filename {
		checksum = ((checksum & 1) << 7) + (checksum >> 1) + b
	}
	write(f, checksum) // Checksum of the short name
	f.Write([]byte{'.', 0, 't', 0, 'x', 0, 't', 0, 0, 0, 0xff, 0xff})
	write(f, uint16(0x0)) // Dummy first cluster
	f.Write([]byte{0xff, 0xff, 0xff, 0xff})

	// Short filename entry
	f.Write(filename)
	write(f, uint8(0x0)) // No File Attributes
	write(f, uint8(0x0)) // No Additional Flags
	write(f, uint8(0x0)) // Dummy fine create time
	write(f, uint16(0x0)) // Dummy create time
	write(f, uint16(0x0)) // Dummy create date
	write(f, uint16(0x0)) // Dummy access date
	write(f, uint16(0x0)) // MSBs of first cluster id
	write(f, uint16(0x0)) // Dummy last modified time
	write(f, uint16(0x0)) // Dummy last modified date
	write(f, uint16(0x3)) // LSBs of first cluster id
	write(f, uint32(0x6)) // File size

	//// First cluster of HELLO.TXT
	seek(f, 4, 0)
	f.Write([]byte{'H', 'e', 'l', 'l', 'o', '\n'})

	// Pad out the rest of the HELLO.TXT data cluster
	seek(f, 5, -1)
	write(f, uint8(0))

	return nil
}

func write(w io.Writer, data interface{}) error {
	return binary.Write(w, binary.LittleEndian, data)
}

func seek(f *os.File, sectorNum, offset int64) error {
	_, err := f.Seek((bytesPerSector * sectorNum) + offset, 0)
	return err
}